Numerical modeling of gas recovery from methane hydrate reservoirs

by 1974- Silpngarmlert, Suntichai

Class 1 hydrate deposits are characterized by a hydrate bearing layer underlain by a two phase, free-gas and water, zone. A Class 1 hydrate reservoir is more preferable than class 2 and class 3 hydrate accumulations because a small change of pressure and temperature can induce hydrate dissociation. In this study, production characteristics from class 1 methane-hydrate reservoirs by means of conventional depressurization technique are studied. In this work, the production characteristics and efficiency from different production strategies (mainly focused on a constant bottom-hole pressure production scheme) such as well-completion locations, well spacing, and production scheduling are investigated. In the production of conventional gas reservoirs using a constant bottom-hole pressure production scheme, both gas and water production rates exponentially decrease with time. However, for methane-hydrate reservoirs, gas production rate exponentially declines with time whereas water production rate increases with time because methane hydrate dissociation increases water saturation of the reservoir. The effects of well-completion locations on the production performances are examined. The simulation results indicate that the moving well completion location strategy provides better gas production performance than the fixed completion location strategy. The optimum well-completion location (using a moving completion location strategy) is at the middle of free-gas zone. Due to the effects of hydrate saturation on formation permeability, one should not complete a well in the hydrate zone. iii The effect of well spacing on the production efficiency is also investigated. As expected, smaller well-spacing system yields more total gas production and it can dissociate gas-hydrate more rapidly than the larger well-spacing system. However, the number of wells increases when the well-spacing decreases resulting in the increase of the capital investment of the project. Based on this study, when the well-spacing increased about 100 percent (from 45.0 acres to 74.38 acres) the cumulative gas production decreased about 8.4 percent at 1,000 days of production. Therefore, once the similar simulation study for a particular reservoir has been performed, the optimum well spacing for a specific reservoir can be determined. The effect of well scheduling on the production performance is also examined. In multiple-well systems, starting all production wells at the same time provides faster hydrate dissociation. However, based on this study, starting production wells at different times yields more produced gas (about 10 percent by volume) even though less gashydrate dissociates. Therefore, starting production wells in the multiple-well system at different times could help in improving the gas production efficiency. iv